Stabilization of dicarbonate diesters with protonic acids

ABSTRACT

By using protonic acids, diesters of dicarbonic acid may be stabilized against thermal and chemical decomposition over a relatively long period. Mixtures of diesters of dicarbonic acid and protonic acids are outstandingly suitable for preserving foods.

The present invention relates to the use of protonic acids asstabilizers of diesters of dicarbonic acid, mixtures containing diestersof dicarbonic acid and protonic acids, and also the use of thesemixtures for preserving foods, drinks and materials.

Diesters of dicarbonic acid are used, inter alia, for preserving foods,in particular drinks, as components of antimicrobial reagents, fordeactivating enzymes in fermentation processes, or for the synthesis offine chemicals or polymers. Diesters of dicarbonic acid are used, inaddition, for example as catalysts for the oxidation of amines, or forsynthesis, for example in the introduction of protecting groups.

It is known that the stability of diesters of dicarbonic acid can berelatively low at room temperature, and in particular at elevatedtemperature. In particular during purification, for example purificationby distillation, or during relatively long storage, decomposition ofdiesters of dicarbonic acid can therefore occur. This decomposition canimpair the quality and purity of the diesters of dicarbonic acid. Inaddition, the decomposition generally proceeds the more rapidly the moreimpurities are present. High purity and stabilization of diesters ofdicarbonic acid are therefore highly desirable.

Methods for improving the thermal stability of diesters of dicarbonicacid are already known from the prior art. For instance, it is proposed,for example, to stabilize dialkyl dicarbonates by adding metal sulphates(cf. JP-A 48-4016). A disadvantage of this method, however, is thatthese metal sulphates are sparingly to poorly miscible with the dialkyldicarbonates.

In addition, it is known to stabilize dialkyl dicarbonates by addingboron compounds (cf. JP-A 46-37810). However, a disadvantage in thiscase is the toxicity of the corresponding boron compounds. Usage infoods does not come into consideration for these additions.

In addition, carbonyl compounds and also heteroanalogous carbonylcompounds have been proposed as additives increasing the storagestability of solutions of dialkyl dicarbonates in solvents inert todialkyl dicarbonates (cf. DE-A 3 231 397). However, solutions of dialkyldicarbonates in customary aprotic solvents scarcely come intoconsideration as an addition to foods. In addition, stabilizing effectsmay only be achieved using relatively high percentage amounts ofadditions.

There was therefore a requirement for stabilizers which are suitable forprotecting diesters of dicarbonic acid effectively against thermalbreakdown.

Surprisingly, it has now been found that diesters of dicarbonic acid canbe stabilized by addition of various protonic acids against thermaland/or chemical breakdown reactions such as can occur, for example instorage or purification by distillation.

The present invention therefore relates to the use of at least oneprotonic acid for stabilization of diesters of dicarbonic acid againstchemical and/or thermal breakdown reactions.

The diesters of dicarbonic acid are preferably compounds of the generalformula (I)

where

-   R¹ and R² independently of one another are straight-chain or    branched C₁-C₈-alkyl, cycloalkyl, C₂-C₈-alkenyl, C₂-C₈-alkynyl or    benzyl,    each of which is optionally monosubstituted to polysubstituted,    identically or differently by halogen; nitro; cyano; C₁-C₆-alkoxy;    dialkylamino; or are phenyl which is optionally monosubstituted to    polysubstituted, identically or differently by halogen; nitro;    cyano; alkyl; haloalkyl; alkoxy; haloalkoxy; acyl; acyloxy;    alkoxycarbonyl; carboxyl,    preferably-   R¹ and R² independently of one another are straight-chain or    branched C₁-C₈-alkyl or C₂-C₈-alkenyl or benzyl,    particularly preferably-   R¹ and R² independently of one another are straight-chain or    branched C₁-C₅-alkyl or C₃-alkenyl or benzyl,    and very particularly preferably-   R¹ and R² independently of one another are methyl, ethyl, isopropyl,    tert-butyl, tert-amyl, allyl or benzyl.

The stabilizers of the invention are protonic acids of different acidstrength.

Preferred protonic acids which come into consideration are, for example,the inorganic acids frequently used in industry and derivatives thereof,and also organic carboxylic acids and derivatives thereof.

Particularly preferred inorganic acids are hydrochloric acid, sulphuricacid, sulphurous acid, nitric acid, nitrous acid, hypochlorous acid,chlorous acid, chloric acid, perchloric acid and the like. The acids arecustomarily used as aqueous solutions.

Examples of derivatives of inorganic acids (sulphuric acid andsulphurous acid) which may be mentioned are particularly sulphonicacids, sulphinic acids and sulphamic acids. Particularly preferredderivatives from the group of the sulphonic acids are, for example,alkylsulphonic acids, phenylsulphonic acids, methylsulphonic acid,fluorosulphonic acid and strongly acidic ion exchangers as aredisclosed, for example, by U.S. Pat. No. 6,646,017. Particularlypreferred derivatives from the group of sulphamic acids are, forexample, cyclohexanesulphamic acids.

Examples of organic carboxylic acids which may be mentioned are:saturated and monounsaturated or polyunsaturated aliphaticmonocarboxylic acids, saturated and mono-unsaturated or polyunsaturatedaliphatic dicarboxylic and polycarboxylic acids. Examples of derivativesof organic carboxylic acids which may be mentioned are: substitutionproducts thereof such as hydroxycarboxylic acids, amino acids, aldehydeand keto acids, derivatives thereof such as carboxylic esters,carboxamides, carbonitriles and hydroxamic acids, and precursors thereofsuch as carbonyl halides, carboxylic anhydrides and ketenes.

Particularly preferred organic carboxylic acids are formic acid, aceticacid, propionic acid, butyric acid, valeric acid, caproic acid,heptanoic acid or longer-chain acids such as fatty acids. Likewiseparticular preference is given to those of the group of polycarboxylicacids, oxalic acid, malonic acid, succinic acid, maleic acid or glutaricacid and also derivatives thereof, for example monoesters thereof, suchas methyl or ethyl esters. Likewise particular preference is given tothose from the group of aliphatic monocarboxylic, dicarboxylic andpolycarboxylic acid derivatives which additionally bear further OHgroups on the carbon backbone and derivatives thereof, such as, forexample citric acid, tartaric acid, malic acid, lactic acid or ascorbicacid. In this case the polycarboxylic acids can be present as partialalkyl esters and in addition the OH groups can be alkylated or likewiseesterified. Also, further organic radicals can be bound via the OHfunctionalities to the carboxylic acids.

The said carboxylic acids can also be used in the form of their saltssuch as, for example, the sodium, potassium, magnesium or calcium salts.

If the abovementioned carboxylic acids have an asymmetric carbon atom,not only the pure enantiomers but also the enantiomeric ordiastereomeric mixture may be used.

The protonic acids can be used as pure substances or as aqueous oralcoholic solutions. The compounds can equally be dissolved in advancein diesters of dicarbonic acid or other suitable solvents. The protonicacids can also be immobilized on surfaces.

In addition, of course, use can be made of the particular reactiveprecursors of the acids which in the presence of water hydrolyse, insitu, to give the abovementioned protonic acids. Examples of these arefor instance acid chlorides or acid anhydrides.

The said stabilizers are generally used in an amount of 0.01 to 100 000ppm, preferably in an amount of 0.1 to 10 000 ppm, particularlypreferably in an amount of 0.1 to 3000 ppm, very particularly preferablyin an amount of 0.1 to 2000 ppm, based on the diesters of dicarbonicacid or mixture thereof.

As a result of the use according to the invention, it is possible tostabilize diesters of dicarbonic acid in general against thermal andchemical breakdown reactions. Such breakdown reactions occur, forexample, in storage.

The diesters of dicarbonic acid stabilized according to the inventionare distinguished by improved storage stability. For instance, thediesters of dicarbonic acid stabilized in this manner can be stored fora plurality of months at room temperature without decomposition of thediesters of dicarbonic acid being observed.

The present invention further relates to mixtures containing one or morediesters of dicarbonic acid of the formula (I) illustrated above and oneor more of the above generally and preferably described protonic acidsgenerally in an amount of 0.01 to 100 000 ppm, preferably in an amountof 0.1 to 10 000 ppm, particularly preferably in an amount of 0.1 to3000 ppm, very particularly preferably in an amount of 0.1 to 2000 ppm,based on the diesters of dicarbonic acid or mixture thereof.

Very particular preference is given to mixtures of at least one diesterof dicarbonic acid of the formula (I), in particular dimethyldicarbonate and/or diethyl dicarbonate with one or more protonic acidsfrom the series of the inorganic acids described as preferred andparticularly preferred, and derivatives thereof and the aliphaticmonocarboxylic, dicarboxylic and polycarboxylic acids which additionallybear further OH groups on the carbon backbone, such as, for example,citric acid, tartaric acid, malic acid, lactic acid or ascorbic acid.

The inventive mixtures can be stored over a period of a plurality ofmonths without decomposition of the diesters of dicarbonic acid presenttherein occurring.

The inventive mixtures are outstandingly suitable for preserving foodsand in particular drinks against infection and/or decomposition bymicroorganisms, such as, for example, bacteria, fungi or yeasts.

The present invention likewise relates to the use of the inventivemixtures for preservation of foods and drinks.

The diesters of dicarbonic acid stabilized according to the inventionare outstandingly suitable, for example, as cold disinfectants for stillor carbonated drinks such as soft drinks, vitamin drinks, fruit juicedrinks, tea drinks, alcoholic or dealcoholized wine drinks, fruitpunches or some beers. Customarily, for this the diesters of dicarbonicacid are added in amounts between 10 and 250 ppm close in time topackaging the drinks. Admixture to the drinks is performed using specialmetering pumps. The diesters of dicarbonic acid act so as to control aseries of microorganisms such as fermentative yeasts, moulds orfermentative bacteria. Examples which may be mentioned here are forinstance Saccharomyces cerevisiae, Mycoderma, Brettanomyces spp,Lactobacillus brevis, Lactobacillus buchneri and many others.

Thermal breakdown reactions of diesters of dicarbonic acid also occur,furthermore, in particular in the purification, e.g. in the extractionor distillation of diesters of dicarbonic acid as carried out, forexample, in the context of the production method for diesters ofdicarbonic acid. By means of the inventive use of protonic acids it ispossible to distil diesters of dicarbonic acid with relatively lowlosses and in relatively high purity.

The present invention therefore further relates to a method for thepurification by distillation of diesters of dicarbonic acid, by admixingone or more diesters of dicarbonic acid of the above-specified formula(I) with one or more of the above generally preferably and particularlypreferably mentioned protonic acids, generally in an amount of 0.01 to100 000 ppm, preferably in an amount of 0.1 to 10 000 ppm, in each casebased on the diesters of dicarbonic acid or mixture thereof, andsubsequently distilling the mixture at a pressure of 5 to 100 mbar,preferably 10-50 mbar, and a temperature between 30 and 120° C.,preferably between 40 and 90° C. Distillation columns customary inindustry come into consideration for the distillation.

The yields of diesters of dicarbonic acid in the distillation arecustomarily >99%.

The present invention further relates to the stabilization of diestersof dicarbonic acid by setting an elevated, compared with high-puritydiesters of dicarbonic acid, proton concentration to a pH of less than6.5, preferably a pH of 6.0 to minus 5.0, and particularly preferably of5.5 to 0. This can be achieved, for example, by the inventive additionof protonic acids in amounts from 0.01 to 100 000 ppm.

The proton concentration is measured in aqueous media customarily viathe oxonium ions forming therein. As a conventional characteristic,therefore the pH is defined. The measurement of the pH can proceed, forexample, after suitable sample preparation, via titration with suitablebases. The end point of the titration is indicated customarily via acolour change of an indicator dye. The pH can, however, also bemeasured, for example, by an electrochemical method. Here, customarily,use is made of pH electrodes, what are termed one-rod electrodes.

In organic liquids, depending on moisture content, likewise a pH can bemeasured, frequently very reproducibly, using pH electrodes. Anotherpossibility, in the case of organic liquids, of measuring the protonconcentration comprises a sample preparation. For example, in the caseof diesters of dicarbonic acid, the proton concentration can bedetermined after extraction with ultrapure water. The diesters ofdicarbonic acid are admixed as organic liquids with a sparing amount ofwater, well mixed, and the phases separated. From the pH of the aqueousphase, the amount of acid originally present in the diester ofdicarbonic acid can be calculated.

The examples hereinafter serve to illustrate the subject matter of thepresent invention without, however, restricting it thereto.

EXAMPLES Example 1

Corresponding to the data in Tables 1-4, in each case defined amounts ofa defined high-purity diester of dicarbonic acid and the respectivelystated additions were weighed in a 10 ml round-bottomed flask equippedwith a magnetic stirrer. The exact amounts of the additions used in eachcase are likewise given in the tables.

The round-bottomed flask was tightly closed by a septum. In this septumwas situated an orifice in which a Teflon tube was attached, which waspassed into a vertical silicone-oil-filled 50 ml burette calibrated to0.1 ml. On the scale of the burette, the amount of the carbon dioxidedeveloping as a result of the decomposition of the diester of dicarbonicacid could be read off. The flask was promptly lowered into a constanttemperature oil bath (stirred at 500 rpm) as specified in Tables 1-4 forthe respective experiment. The depth of immersion of the flask was 2.0cm.

After the respectively stated time, generally after 1, 2, 5, 10 and 15minutes, the gas volume was read off. The gas volume is an index of thedegree of decomposition of the diesters of dicarbonic acid to give CO₂.It thus inversely reflects the degree of stabilization by the additionstested.

In most cases the experiments were repeated in order to ensurereproducibility. Meaningful reproducibility was present in each case.

The results may be taken from the appended tables. High-purity diesterof dicarbonic acid, in the observed time, released little carbondioxide, but even contact with small amounts of silica gel, manganesedioxide or else only rough surfaces such as scratched glass drasticallyaccelerated decomposition. Small amounts of the stabilizers weresufficient for effective reduction of the decomposition.

The fewer gaseous decomposition products diesters of dicarbonic acidrelease under temperature stress, the more favourably does distillationunder vacuum proceed.

TABLE 1 Dimethyl dicarbonate, 1670 ppm addition Temperature 100 100 100100 100 100 100 Dimethyl dicarbonate 3 3 3 3 3 3 3 Amount [g] Additionwithout silica gel silica gel silica gel silica gel silica gel silicagel Amount [mg] 10 10 10 10 10 10 Addition without without water H₂SO₄HCL 37.0% HNO₃ 65.0% perchloric acid Amount in mg 5 5 5 5 5 Gasevolution [ml] Minute 1 0.1 1.0 2.4 0.1 0.6 0.5 0.2 Minute 2 0.2 3.4 8.00.3 1.6 1.4 1.0 Minute 5 0.6 20.3 26.8 1.9 30.0 2.5 10.9 Minute 10 0.846.1 50.0 4.9 4.2 3.5 17.1 Minute 15 1.3 50.0 50.0 10.2 6.1 4.5 23.1Temperature 100 100 100 100 100 100 100 Velcorin 3 3 3 3 3 3 3 Amount[g] Addition silica gel silica gel silica gel silica gel silica gelsilica gel silica gel Amount [mg] 10 10 10 10 10 10 10 Addition ascorbiccitric L(−)malic D(−)-tartaric L(+)-tartaric N-cyclohexyl- acetic acid +acid acid acid acid 99% acid 99.5% p.a sulphamic acid H₂SO₄ Amount in mg5 5 5 5 5 5 2.5 + 2.5 Gas evolution [ml] Minute 1 0.5 0.6 0.1 1.0-0.50.2-0.3 0.2 0.1 Minute 2 1.8 1.5 0.2 2.2-1.5 0.6-0.6 1.0 0.4 Minute 58.2 2.5 0.4 3.3-2.6 0.9-1.1 9.6 1.5 Minute 10 23.9 3.7 0.9 4.2-3.01.0-1.2 16.1 3.4 Minute 15 33.6 6.4 2.7 4.7-3.7 1.1-1.5 19.6 6.4Dimethyl dicarbonate, 1670 ppm addition of stabilizer Temperature [° C.]100 100 100 100 100 100 100 Dimethyl dicarbonate 3 3 3 3 3 3 3 Amount[g]Addition without silica gel silica gel silica gel silica gel silica gelsilica gel Amount [mg] 10 10 10 10 10 10 Addition of stabilizer withoutwithout water H₂SO₄ HCL HNO₃ perchloric 98.0% 37.0% 65.0% acid Amount[mg] 5 5 5 5 5 Gas evolution [ml] Minute 1 0.1 1.0 2.4 0.1 0.6 0.5 0.2Minute 2 0.2 3.4 8.0 0.3 1.6 1.4 1.0 Minute 5 0.6 20.3 26.8 1.9 3.0 2.510.9 Minute 10 0.8 46.1 50.0 4.9 4.2 3.5 17.1 Minute 15 1.3 50.0 50.010.2 6.1 4.5 23.1 Temperature [° C.] 100 100 100 100 100 100 100Dimethyl dicarbonate 3 3 3 3 3 3 3 Amount [g] Addition silica gel silicagel silica gel silica gel silica gel silica gel silica gel Amount [mg]10 10 10 10 10 10 10 Addition of stabilizer ascorbic citric acidL(−)-malic D(−)- L(+)- N-cyclohexyl- acetic acid + acid acid tartarictartaric sulphamic H₂SO₄ acid acid acid 98% 99%. 99.5% p.a. Amount [mg]5 5 5 5 5 5 2.5 + 2.5 Gas evolution [ml] Minute 1 0.5 0.6 0.1 1.0 0.20.2 0.1 Minute 2 1.8 1.5 0.2 2.2 0.6 1.0 0.4 Minute 5 8.2 2.5 0.4 3.30.9 9.6 1.5 Minute 10 23.9 3.7 0.9 4.2 1.0 16.1 3.4 Minute 15 33.6 6.42.7 4.7 1.1 19.6 6.4 Temperature [° C.] 100 100 100 Dimethyl dicarbonate3 3 3 Amount [g] Addition silica gel silica gel silica gel Amount [mg]10 10 10 Addition of stabilizer benzenesulphonic toluenesulphonicmethanesulphonic acid acid acid Amount [mg] 5 5 5 Gas evolution [ml]Minute 1 0.5 0.7 0.6 Minute 2 1.4 1.6 1.1 Minute 5 2.5 2.6 2.0 Minute 103.8 4.1 2.4 Minute 15 6.0 5.3 3.2

TABLE 2 Dimethyl dicarbonate, 1670 ppm addition of stabilizerTemperature [° C.] 100 100 100 100 100 100 100 Dimethyl 3 3 3 3 3 3 3dicarbonate Amount [g] Addition Manganese Manganese Manganese ManganeseManganese Manganese Manganese dioxide dioxide dioxide dioxide dioxidedioxide dioxide Amount [mg] 10 10 10 10 10 10 10 Addition of without HClHNO₃ citric acid L(−)-malic acid L(+)-tartaric acid water stabilizer37.0% 65.0% 99.5% p.a. Amount [mg] 5 5 5 5 5 5 Gas evolution [ml] Minute1 3.8 0.9 0.9 2.5 0.5 2.9 7.1 Minute 2 9.3 2.0 2.8 5.9 1.2 6.2 26.4Minute 5 21.7 6.9 7.4 8.8 2.7 9.0 35.4 Minute 10 31.1 9.6 13.6 11.7 3.910.5 46.1 Minute 15 41.5 12.2 19.6 15.9 5.3 11.2 50.0

TABLE 3 Dimethyl dicarbonate, 1670 ppm addition of stabilizerTemperature [° C.] 100 100 100 Dimethyl dicarbonate 3 3 3 Amount [g]Addition without surface of the flask surface of the highly internallyflask highly scratched internally scratched Addition of stabilizerwithout without L(+)-tartaric acid Amount [mg] 5 Gas evolution [ml]Minute 1 0.1 0.7 0.6 Minute 2 0.2 1.3 1.4 Minute 5 0.5 2.2 2.1 Minute 101.3 3.8 2.7 Minute 15 2.7 5.7 3.0

TABLE 4 Dimethyl dicarbonate, <1000 ppm addition of stabilizerTemperature 100 100 100 100 100 [° C.] Dimethyl 3 3 3 3 3 dicarbonateAmount [g] Addition silica gel silica silica gel silica gel silica gelgel Amount [mg] 10 10 10 10 10 Addition of 100 ppm 10 ppm 100 ppm 100ppm 100 ppm stabilizer malic H₂SO₄ H₂SO₄ L(+)- HNO₃ acid 98.0% 98.0%tartaric 65.0% acid Gas evolution [ml] Minute 1 0.2 0.7 0.9 0.4 0.2Minute 2 1.4 1.9 1.5 1.7 0.6 Minute 5 14.3 11.2 2.7 16.1 2.5 Minute 1040.4 33.5 4.2 38.7 34.4 Minute 15 15.5

Example 2

The association between stabilization and the pH determined directly bya pH electrode was measured experimentally. High-purity dimethyldicarbonate was first admixed with different amounts of acid or base.The pH then established was measured directly using a pH electrode(one-rod electrode) from Mettler Toledo, Model Inlab 1010. The electrodewas calibrated in advance in buffer solutions. Thereupon the instabilitywas determined experimentally as a function of pH in a similar manner tothe procedure in Example 1. The respective values for carbon dioxideevolution may be found in the table. A clear correlation can be seenbetween proton content and stability. Likewise, the amount of inorganicacid present in the diester of dicarbonic acid after extraction withultrapure water was determined. For this the dimethyl dicarbonate wasextracted with a sparing amount of high-purity water and the pH measuredin this water using the glass electrode before or after phaseseparation. Thereafter the pH was converted to the original dicarbonatevolume. This procedure gave identical values and less drift of the pHelectrode. The solutions were flushed with argon during the pHdetermination in order to prevent the pH from being influenced by carbondioxide.

Dimethyl dicarbonate, pH dependence Temperature [° C.] 100 100 100 100100 100 100 100 100 100 Dimethyl 3 3 3 3 3 3 3 3 3 3 dicarbonate Amount[g] Addition silica silica gel 1 silica gel 1 silica gel 1 silica gel 1silica gel 1 silica gel 1 silica gel 1 silica gel 1 silica gel 1 gel 1Amount [mg] 10 10 10 10 10 10 10 10 10 10 Measured pH −1 0.6 2.6 3.9 4.25.9 6.3 7.9 8.4 9.5 Gas evolution [ml] Minute 1 0.3 0.1 1.1 0.3 0.7 0.41.0 1.9 3.3 6.4 Minute 2 0.6 0.4 2.2 1.2 1.9 1.8 7.8 18.2 30.9 42.5Minute 5 1.0 1.0 5.3 5.6 13.4 13.3 29.6 49.2 50.0 50.0 Minute 10 1.4 1.98.2 22.2 37.3 37.2 50.0 50.0 50.0 50.0 Minute 15 1.8 3.3 10.9 35.2 50.050.0 50.0 50.0 50.0 50.0

1. Use of at least one compound from the series of protonic acids forstabilization of diesters of dicarbonic acid against chemical andthermal breakdown reactions.
 2. Use according to claim 1, characterizedin that the protonic acids are inorganic acids and derivatives thereof,and also organic carboxylic acids and derivatives thereof,
 3. Useaccording to at least one of claims 1 and 2, characterized in that theinorganic acids are hydrochloric acid, sulphuric acid, sulphurous acid,nitric acid, nitrous acid, hypochlorous acid, chlorous acid, chloricacid, perchloric acid.
 4. Use according to at least one of claims 1 and2, characterized in that the derivatives of the inorganic acids arealkylsulphonic acids, phenylsulphonic acids, methylsulphonic acid,fluorosulphonic acid and strongly acidic ion exchangers.
 5. Useaccording to at least one of claims 1 and 2, characterized in that theorganic carboxylic acids are saturated and monounsaturated orpolyunsaturated aliphatic monocarboxylic acids and saturated andmonounsaturated or polyunsaturated aliphatic dicarboxylic andpolycarboxylic acids and derivatives thereof, carbonyl halides, carbonicanhydrides, carbonic esters, carboxamides, carbonitriles, hydroxamicacids, ketenes, hydroxycarboxylic acids, amino acids, aldehyde and ketoacids.
 6. Use according to claim 5, characterized in that the organiccarboxylic acids are formic acid, acetic acid, propionic acid, butyricacid, valeric acid, caproic acid, heptanoic acid, longer-chain saturatedand unsaturated fatty acids, oxalic acid, malonic acid, succinic acid,maleic acid, glutaric acid, citric acid, tartaric acid, malic acid,lactic acid or ascorbic acid or derivatives thereof.
 7. Use according toat least one of claims 1 to 6, characterized in that the diesters ofdicarbonic acid are compounds of the general formula

where R¹ and R² independently of one another are straight-chain orbranched C₁-C₈-alkyl, cycloalkyl, C₂-C₈-alkenyl, C₂-C₈-alkynyl orbenzyl, each of which is optionally monosubstituted to polysubstituted,identically or differently by halogen; nitro; cyano; C₁-C₆-alkoxy;dialkylamino; or are phenyl which is optionally monosubstituted topolysubstituted, identically or differently by halogen; nitro; cyano;alkyl; haloalkyl; alkoxy; haloalkoxy; acyl; acyloxy; alkoxycarbonyl;carboxyl.
 8. Use according to at least one of claims 1 to 7,characterized in that the diesters of dicarbonic acid are dimethyldicarbonate or diethyl dicarbonate.
 9. Use according to at least one ofclaims 1 to 8, characterized in that the protonic acid or mixturethereof is used in an amount of 0.01 to 100 000 ppm, based on dialkyldicarbonates or mixture thereof.
 10. Use according to at least one ofclaims 1 to 9, characterized in that the stabilization against breakdownreactions is during workup, extraction, distillation or storage. 11.Mixtures containing one or more diesters of dicarbonic acid of thegeneral formula

where R¹ and R² independently of one another are straight-chain orbranched C₁-C₈-alkyl, cycloalkyl, C₂-C₈-alkenyl, C₂-C₈-alkynyl orbenzyl, each of which is optionally monosubstituted to polysubstituted,identically or differently by halogen; nitro; cyano; C₁-C₆-alkoxy;dialkylamino; or are phenyl which is optionally monosubstituted topolysubstituted, identically or differently by halogen; nitro; cyano;alkyl; haloalkyl; alkoxy; haloalkoxy; acyl; acyloxy; alkoxycarbonyl;carboxyl, and one or more protonic acids in an amount of 0.01 to 100 000ppm, based on dialkyl dicarbonates or mixture thereof.
 12. Mixtureaccording to claim 11, characterized in that protonic acids areinorganic acid and derivatives thereof and also organic carboxylic acidsand derivatives thereof.
 13. Mixture according to at least one of claims11 and 12, characterized in that the inorganic acids and derivativesthereof are hydrochloric acid, sulphuric acid, sulphurous acid, nitricacid, nitrous acid, hypochlorous acid, chlorous acid, chloric acid,perchloric acid, sulphonic acids or strongly acidic ion exchangers. 14.Mixture according to at least one of claims 11 and 12, characterized inthat the organic carboxylic acids and derivatives thereof are saturatedand monounsaturated or polyunsaturated aliphatic monocarboxylic acids,saturated and monounsaturated or polyunsaturated aliphatic dicarboxylicand polycarboxylic acids and derivatives thereof are carbonyl halides,carboxylic anhydrides, carboxylic esters, carboxamides, carbonitriles,hydroxamic acids, ketenes, hydroxycarboxylic acids, amino acids,aldehyde and keto acids.
 15. Mixture according to at least one of claims11 to 14, characterized in that it is at least one compound from theseries dimethyl dicarbonate and diethyl dicarbonate and at least oneprotonic acid from the series hydrochloric acid, sulphuric acid,sulphurous acid, nitric acid, nitrous acid, hypochlorous acid, chlorousacid, chloric acid, perchloric acid, formic acid, acetic acid, propionicacid, butyric acid, valeric acid, caproic acid, heptanoic acid,longer-chain saturated and unsaturated fatty acids, oxalic acid, malonicacid, succinic acid, maleic acid, glutaric acid, citric acid, tartaricacid, malic acid, lactic acid or ascorbic acid or derivatives thereof.16. Use of a mixture according to at least one of claims 11 to 15 forpreservation of foods, drinks and materials.
 17. Method for thepurification by distillation of dialkyl dicarbonates, characterized inthat one or more dialkyl dicarbonates of the general formula

where R¹ and R² independently of one another are straight-chain orbranched C₁-C₈-alkyl, cycloalkyl, C₂-C₈-alkenyl, C₂-C₈-alkynyl orbenzyl, each of which is optionally monosubstituted to polysubstituted,identically or differently by halogen; nitro; cyano; C₁-C₆-alkoxy;dialkylamino; or are phenyl which is optionally monosubstituted topolysubstituted, identically or differently by halogen; nitro; cyano;alkyl; haloalkyl; alkoxy; haloalkoxy; acyl; acyloxy; alkoxycarbonyl;carboxyl, is admixed with one or more protonic acids in an amount of0.01 to 100 000 ppm, based on dialkyl dicarbonates or mixture thereofand the mixture is subsequently distilled.
 18. Method according to claim17, characterized in that the protonic acids are inorganic acids andderivatives thereof and also organic carboxylic acids and derivativesthereof.
 19. Method according to at least one of claims 17 and 18,characterized in that the protonic acids are those of the serieshydrochloric acid, sulphuric acid, sulphurous acid, nitric acid, nitrousacid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid,sulphonic acids, formic acid, acetic acid, propionic acid, butyric acid,valeric acid, caproic acid, heptanoic acid, longer-chain saturated andunsaturated fatty acids, oxalic acid, malonic acid, succinic acid,maleic acid, glutaric acid, citric acid, tartaric acid, malic acid,lactic acid or ascorbic acid or derivatives thereof.
 20. Methodaccording to at least one of claims 17 to 19, characterized in thatdistillation is carried out at a pressure of 5 to 100 mbar and atemperature between 30 and 120° C.
 21. Diester of dicarbonic acid of thegeneral formula

where R¹ and R² independently of one another are straight-chain orbranched C₁-C₈-alkyl, cycloalkyl, C₂-C₈-alkenyl, C₂-C₈-alkynyl orbenzyl, each of which is optionally monosubstituted to polysubstituted,identically or differently by halogen; nitro; cyano; C₁-C₆-alkoxy;dialkylamino; or are phenyl which is optionally monosubstituted topolysubstituted, identically or differently by halogen; nitro; cyano;alkyl; haloalkyl; alkoxy; haloalkoxy; acyl; acyloxy; alkoxycarbonyl;carboxyl, with a pH of less than 6.5.